The present invention relates to methods for start up of pots or cells used in the electrolytic production of metals. More specifically, the present invention relates to methods for preheating molten salt electrolysis cells so as to protect electrodes, especially inert anodes and their support structure assemblies, from thermal shock and exposure to products of combustion.
Aluminum is produced conventionally by the electrolysis of alumina dissolved in cryolite-based molten electrolytes at temperatures between about 900xc2x0 C. and 1000xc2x0 C.; the process is known as the Hall-Heroult process. A Hall-Heroult reduction cell typically comprises a steel shell having an insulating lining of refractory material, which in turn has a lining of carbon that contacts the molten constituents. Conductor bars connected to the negative pole of a direct current source are embedded in the carbon cathode substrate that forms the cell bottom floor. The carbon lining and cathode substrate have a useful life of three to eight years, or even less under adverse conditions. The deterioration of the cathode bottom is due to erosion and penetration of electrolyte and liquid aluminum as well as intercalation of sodium, which causes swelling and deformation of the cathode carbon blocks and ramming mix. In addition, the penetration of sodium species and other ingredients of cryolite or air leads to the formation of toxic compounds including cyanides. Anodes are at least partially submerged in the bath.
In operation, the conventional cell contains an electrolytic, molten cryolite-based bath in which alumina is dissolved. A molten aluminum pool acts as the cathode. A crust of frozen electrolyte and alumina forms on top of the bath and around the anode blocks. As electric current passes through the bath between the anode and cathode surfaces, alumina is reduced to aluminum, which is deposited in the pad of molten metal.
Electrolytic reduction cells must be heated from room temperature to approximately the desired operating temperature before the production of metal can be initiated. Heating should be done gradually and evenly to avoid thermal shock, which can in turn cause breakage or spalling of the anodes, sidewalls and cathode blocks. The heating operation minimizes thermal shock to the lining and the electrodes upon introduction of the molten electrolyte to the cell. Thermal gradients as low as 50xc2x0 C. can cause cracking.
Preheating of cells is typically performed by either a gas preheat or by resistor block. The gas pre-heating step results in the generation of products of combustion (POC), such as CO, CO2, and H2O, which can be deleterious to the inert anodes. CO can reduce the oxides in the anodes, eventually leading to corrosion. CO2 and H2O can oxidize metallic constituents of the anode, again leading to corrosion. It is therefore desirable to protect the anodes from all constituents present in POC.
Aluminum electrolysis cells have historically employed carbon anodes on a commercial scale. The energy and cost efficiency of aluminum smelting can be significantly reduced with the use of inert, non-consumable, and dimensionally stable anodes. Use of inert anodes rather than traditional carbon anodes allows a highly productive cell design to be utilized, thereby reducing capital costs. Significant environmental benefits are also realized because inert anodes produce essentially no CO2 or CF4 emissions. Some examples of inert anode compositions are provided in U.S. Pat. Nos. 4,374,050; 4,374,761; 4,399,088; 4,455,211; 4,582,585; 4,584,172; 4,620,905; 5,279,715; 5,794,112; 5,865,980; and 6,126,799, assigned to Alcoa Inc. These patents are incorporated herein by reference. Inert anodes can undergo thermal shock if heated or cooled too quickly, and should also be protected from exposure to POC.
The present invention is directed to methods for protecting electrodes, especially inert anodes, during start up of electrolytic cells, or xe2x80x9cpotsxe2x80x9d as they are referred to in the art. Inert anodes should be carefully heated to temperature, and the gas atmosphere during heating around the anodes should be carefully controlled. More specifically, the gas atmosphere during heating should be maintained in a slightly oxidizing state, with enough oxidation to minimize carbon deposit or soot, but with not so much oxidation that exposed carbon in the cathode and/or sidewalls will be attacked. Inert anodes are protected both from thermal shock and from products of combustion (POC) according to the present methods.
The present invention is directed to the use of recuperative heating to pre-heat an electrolytic cell. Significantly, according to the present method exposure of the inert anodes to POC is eliminated. The gradual temperature increases to which the inert anodes, and other cell components, are exposed reduces, if not eliminates, thermal gradients that can cause spalling or breakage of the anodes.